1. Field of the Invention
The present invention relates to organic silicon oxide fine particles which can be formed into a porous film excellent in dielectric properties, mechanical strength and chemical stability by application, a preparation method thereof, a film-forming composition, a formation method of a porous film, a porous film formed thereby, and a semiconductor device having the porous film.
2. Description of the Related Art
In the fabrication of semiconductor integrated circuits, as their integration degree becomes higher, an increase in interconnect delay time due to an increase in interconnect capacitance, which is a parasitic capacitance between metal interconnects, prevents their performance enhancement. The interconnect delay time is called an RC delay which is in proportion to the product of the electric resistance of metal interconnects and the static capacitance between interconnects. Reduction in the resistance of metal interconnects or reduction in the capacitance between interconnects is necessary for reducing this interconnect delay time. The reduction in the resistance of an interconnect metal or interconnect capacitance can prevent even a highly integrated semiconductor device from causing an interconnect delay, which enables size reduction and high speed operation of it and moreover, minimization of power consumption.
In order to reduce the resistance of metal interconnects, semiconductor device structures using copper as metal interconnects have recently replaced those using conventional interconnects made of aluminum. Use of copper interconnects alone, however, has limits in accomplishing performance enhancement so that reduction in the interconnect capacitance is an urgent necessity for further performance enhancement of semiconductor devices.
One method for reducing interconnect capacitance is to reduce the dielectric constant of an interlayer insulating film disposed between metal interconnects. As such a low dielectric constant insulating film, use of a porous film instead of a conventionally used silicon oxide film is now studied. In particular, since a porous film is only one practical film as a material being suited as an interlayer insulating film and having a dielectric constant not greater than 2.5, various methods for forming a porous film have been proposed. When an interlayer insulating film is made porous, however, reduction in mechanical strength and adsorption of water are likely to deteriorate the film so that reduction in dielectric constant (k) by introduction of pores into the film and maintenance of sufficient mechanical strength and hydrophobicity are serious problems that need to be overcome.
A silica film having enhanced mechanical strength can be obtained, for example, by increasing the proportion of tetrafunctional silicon units as a silicon unit constituting the film, thereby constructing a densely crosslinked siloxane structure to form hard particles. In practice, a film obtained by plasma polymerization of tetrafunctional TEOS shows strength as high as 80 GPa in bulk form (form having no porosity). When a film is prepared from a hydrolysis condensate of a trifunctional alkoxysilane having a methyl group, on the other hand, it shows strength of 20 GPa or less even in bulk form (“Low-k Materials and Process Integration after the 65 nm and 45 nm Generations”, by Eiki Shibata, proceedings of a lecture held by Electronic Journal on Apr. 18, 2006, at Ochanomizu/Tokyo). Even if pores are introduced into the above film to decrease their dielectric constant, the strength in bulk form still maintains. Accordingly, it is known that as the proportion of tetrafunctional units becomes larger, high strength can be achieved more easily.
With regard to chemical properties, the binding energy itself of a Si—O bond is greater than that of a Si—C bond so that the former gives a structure resistant to heat decomposition. Difference in reactivity with a chemical substance such as washing fluid is, on the other hand, attributable to a large difference in polarity between the Si—C bond and the Si—O bond. The Si—O bond having a greater polarity is susceptible to the attack (nucleophilic attack) of the chemical substance. Similarly, comparison in polarity between tetrafunctional silicon and trifunctional silicon has revealed that an electron density at the center of tetrafunctional silicon lowers (greater δ+) with the number of Si—O bonds having a large polarity so that it is susceptible to nucleophilic attack. When the number of Si—O bonds decreases as silicon becomes trifunctional or bifunctional, the electron density at the center of the silicon shows a small decrease (smaller δ+). As a result, it is not susceptible to the nucleophilic attack.
When a porous silica film is used as an interlayer insulating film of a semiconductor device, process damage in an etching or washing step poses a problem. In particular, hydrophilization of the surface of the porous silica film after treatment with a washing fluid and moisture absorption resulting therefrom lead to deterioration in the reliability of the semiconductor device. There is therefore a demand for overcoming such a problem.
It has been recognized that the susceptibility of a CVD-LK film to such a process damage becomes smaller with an increase in its carbon content. Also in an LK film of an application type, an increase in carbon content by introducing a carbosilane skeleton is under study (JP 2007-262257A).